The present invention relates generally to communications systems and more particularly to data differentiation in communications systems.
Several modem communications systems employ packet-multiplexed transmission, in which multiple data sources transmit over a shared medium using a time-division multiple-access (TDMA) protocol to a central master location. For example, one such system is a passive fiber optic network described in ITU Standard G.983 (the xe2x80x9cStandardxe2x80x9d), which is incorporated by reference herein in its entirety. FIG. 1 depicts in block diagram form a passive fiber optic network 100 described in the Standard. Multiple remote stations (depicted as ONU 104-1 to 104-n) are data sources that each communicate with a receiver at the master location (the OLT 102). ONUs transmit packets of information during time slots assigned by the OLT, so that packets originating from different ONUs are interleaved in time on the transmission medium. If the ONUs are at different distances from the OLT, the signal level at the OLT can vary substantially depending on which ONU is sending the data. Thus the decision threshold for distinguishing between ONE and ZERO levels in digital data streams must be recalibrated by the OLT every time a different ONU transmits to the OLT because the analog signal strength from each ONU is distinct at the OLT. If different source transmissions are interleaved rapidly on a communication fiber, for example on a packet-by-packet or cell-by-cell basis, rapid threshold recalibrations are necessary.
Establishing a correct threshold value, for example at a level close to the average of the ONE and ZERO levels, is important to minimize impairments due to both amplitude noise and timing errors. If the threshold is placed too close to a ONE or ZERO level, then amplitude noise in the system will increase the frequency of incorrectly detected bits. In addition, improper threshold adjustment causes transitions to occur at incorrect times (so called xe2x80x9cpulse width distortionxe2x80x9d), with the effect that bits of one signal level will have reduced duration while bits at the other logic level will have excessive duration. This pulse width distortion complicates clock recovery and data timing.
Generally, the threshold should be optimally set halfway between the analog signal levels corresponding to digital ONE and ZERO levels. For example, for ATM cells transmitted at 155 megabits/s, each cell is transmitted in 2.7 microseconds, so that the threshold must be reset in a much shorter time (e.g., tens of nanoseconds). As the signaling rate increases above 155 megabits/s, the time to establish the threshold level becomes correspondingly shorter. In the fiber optic passive optical network defined in the Standard, the optical input signal strength to the OLT can vary by as much as 19 dB. Because the extinction ratio (ratio of logical ONE to ZERO levels) is only 10 dB in the Standard, the logical ONE level associated with one ONU may be smaller than the logical ZERO level associated with a different ONU.
FIG. 2 depicts a block diagram of a conventional TDMA receiver 200, used for example by OLT 102, that converts optical signals into binary data. The receiver 200 accepts an analog signal input consisting of a digitally encoded optical signal. The conventional photodiode 202 converts the optical input into an electrical current, and the conventional low-noise trans-impedance amplifier 204 converts the electrical current into an analog voltage. A conventional filter 206, such as a low pass filter, filters the output signal from the amplifier 204 to reduce noise and improve sensitivity, and provides the filtered output to a conventional comparator 212 (also known as a quantizer or limiting amplifier). Threshold setting device 208 is coupled to receive the filtered signal at node 214 from filter 206 and determines and provides a threshold value to the conventional sample-and-hold or track-and-hold amplifier 210. The amplifier 210 provides the threshold value to an input of comparator 212, possibly fixing the value for the duration of an input data packet. The comparator 212 compares the analog voltage against a reference value, the threshold, and outputs a digital signal (either a logic level ONE or logic level ZERO) depending on whether the analog voltage is greater than or less than the threshold.
Typically in the system described in the Standard, data is transmitted by a source in packets. The useful data in a data packet is preceded by a sequence of signal bits (typically a repetitive ONE-ZERO sequence) which carries no useful information. These additional bits, referred to as the preamble, are used to establish the threshold value to be used by the threshold setting device of FIG. 2 for determining the threshold. Because the preamble transports no useful information, the length of the preamble should be minimized to improve the transmission rate of useful data.
One conventional threshold setting device 208 uses a pair of peak detectors to measure the ONE and ZERO levels of the input signal, and then uses the average of these values as the threshold (so called xe2x80x9cpeak detectorxe2x80x9d). See for example the following publications or issued patents which are each incorporated by reference herein in their entirety: M. Nakamura, N. Ishihara, and Y. Akazawa, xe2x80x9cA 156 Mbs CMOS Optical Receiver for Burst-mode Transmissionxe2x80x9d, IEEE J. Sol. St. Circuits, 33, 117901187 (1998); Y. Ota, R. G. Swartz, V. D. Archer, S. K. Korotky, and A. E. Dunlop, xe2x80x9cHigh-speed, Burst-mode Packet Capable Optical Receiver and Instantaneous Recovery for Optical Bus Operationxe2x80x9d, J. Lightwave Technology 12, 325-330 (1994); U.S. Pat. No. 5,475,342, issued Dec. 12, 1995 to Nakamura et al., and entitled xe2x80x9cAmplifier for Stably Maintaining a Constant Outputxe2x80x9d; U.S. Pat. No. 5,430,766, issued Jul. 4, 1995 to Ota et al., and entitled xe2x80x9cBurst Mode Digital Data Receiverxe2x80x9d; and U.S. Pat. No. 5,875,050, issued Feb. 23, 1999 to Ota, and entitled xe2x80x9cBurst Mode Digital Optical Receiverxe2x80x9d.
The peak detector provides very fast threshold detection, but has several difficulties associated with it. For example, the peak detector must operate at the input signal bit rate, which at high rates (e.g., 155 megabits/s or above) requires high slew rate within the peak detection block for accurate peak determination. High slew rate becomes increasingly difficult to achieve at higher bit rates. The peak detector is also fully susceptible to noise corruption because the peak detector must operate at high speed compared to the bit rate to provide high-fidelity signal tracking. Signal noise during peak detection, which can not be filtered, will result in imperfect threshold adjustment. Another example of the difficulty with the peak detector is the cost of peak detection can be higher than other approaches, because it requires two amplifiers (a first for ONE level detection and second for ZERO level detection).
Another conventional threshold setting device uses temporal averaging circuits to measure the average of the ONE and ZERO levels during the preamble (so called xe2x80x9ctemporal averaging detectorxe2x80x9d). The temporal averaging detector uses low pass filters (e.g., resistor-capacitor circuits) or integrators (amplifiers with capacitors) to average the signal level over many bit periods. They generally work by charging a capacitor over many bit periods. See for example, U.S. Pat. No. 5,539,779 issued Jul. 23, 1996 to T. Nagahori, and entitled xe2x80x9cAutomatic offset control circuit for digital receiverxe2x80x9d, which is incorporated by reference herein in its entirety. The temporal averaging detector reduces noise due to the averaging process, but it is typically slow because the preamble must be averaged over many bits. The averaging is typically achieved using a capacitor charging circuit, which can be quite slow. Typical temporal averaging detectors require 24 to 32 bits of preamble.
Thus what is needed is a threshold determination system that provides for rapid and reliable threshold determination at high bit rates.
One embodiment of the present invention includes a system that determines a threshold to distinguish between binary signals, where the system includes a bit wise threshold determination device coupled to receive an input signal and that delays the input signal, averages the input signal and the delayed input signal, and outputs the average to an output node, where the average represents a threshold value.
One embodiment of the present invention includes a system for converting an analog signal into a binary signal, where the system includes: a threshold detection system coupled to receive an input signal, where the threshold detection system delays the input signal, averages the input signal and the delayed input signal, and outputs the average to an output node and where the average represents a threshold signal; an amplifier coupled to receive the threshold signal from the threshold detection system and that outputs a sustained threshold signal; and a comparator coupled to receive the threshold signal from the amplifier and coupled to receive the input signal, where the comparator generates a binary signal based on a comparison between the threshold signal and the input signal.
One embodiment of the present invention includes a system for converting an analog signal into a binary signal, the system including: a threshold detection system coupled to receive an input signal, where the threshold detection system delays the input signal, averages the input signal and the delayed input signal, and outputs the average to an output node and where the average represents a threshold signal; a storage device coupled to receive the threshold signal from the threshold detection system and that stores the threshold signal; and a comparator coupled to receive the threshold signal from the storage device and coupled to receive the input signal, where the comparator generates a binary signal based on a comparison between the threshold signal and the input signal.
Advantageously, embodiments of the present invention determine threshold values more quickly than the temporal averaging detector by measuring threshold values instantaneously as opposed to using an averaging process. Advantageously, embodiments of the present invention determine a threshold value using as few as two (2) bits thereby reducing the required length of a preamble.
This invention will be more fully understood upon consideration of the detailed description below taken together with the accompanying drawings.